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This study determines the influence of mechanical surfacing on tensile shear strength (TSS) and wood failure percentage (WFP) of beech wood (Fagus sylvatica L.) at the wet stage according to prEN 302-1:2011 (tensile shear tests). The wood was planed, sanded and face milled, using different qualities of cutting edges and sanding grits. Roughness and wettability of the adherends were characterised, supplemented by environmental scanning electron microscopy images. The specimens were bonded by means of a one-component polyurethane adhesive and tested at the dry stage (pretreatment A1), at the wet stage (A4) and after re-drying (mA5). Results determined that the wettability and the wood structure's integrity are considered more decisive for the TSS at the wet stage than surface roughness. Planing with very dull knives caused the lowest TSS and WFP after A4 and the most subsurface damage. Face milled and sanded batches revealed the best performance. The TSS and WFP decreased significantly from A1 to A4, but recovered after mA5.

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Tensile shear tests according to EN 302-1 for load-bearing timber structures were performed on European beech wood (Fagus sylvatica L.) and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] bonded by means of a one-component polyurethane adhesive (1C PUR). Results reveal a substantial loss of tensile shear strength (TSS) and wood failure percentage (WFP) at the wet stage compared to the dry stage. As can be seen from microscopic images, this is accompanied by a loss of adhesion at the boundary layer. Therefore, the aim of this work was to find a priming fluid that improves the load transmission between adhesive and adherend at the wet stage without introducing formaldehyde into the gluing process. A substantial improvement of TSS and WFP was achieved by means of the hygroscopic organic solvent N,N-dimethylformamide (DMF). In addition, contact angle measurements were carried out, revealing that DMF heavily enhances the wettability of the joining surface. Furthermore, it was attempted to integrate the outcomes into the swelling strain model stated by Frihart in 2009. By way of comparison a hydroxymethylated resorcinol coupling agent, a mixture of diphenylmethane-4,4 0-diisocyanate isomers and water were also tested as priming fluids. The data confirm that TSS and WFP of 1C PUR bonded wooden joints do not correlate, whilst WFP is mostly not normally (at wet stage often bimodally) distributed.

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